As a method for separating a liquid mixture of water and an organic compound, a distillation method has been known for a long time. However, it is very difficult to separate an azeotropic mixture, a mixture of compounds having close boiling points, or a compound which is liable to be denatured by distillation. Further, even if compounds are separable by distillation, they often require a large amount of energy. In order to solve these problems in a distillation method, various separation methods using polymer membranes have been studied. Among them, pervaporation is considered to be useful for separation of a liquid mixture of water and an organic compound. Pervaporation is a separation method wherein a liquid mixture to be separated is fed to one side of a polymer membrane, while the other side of the membrane is evacuated or maintained at a reduced pressure to preferentially withdraw a permeate in vapor form through the membrane.
The study of this method has been started in the 1950's and, for example, pervaporation has been already disclosed in U.S. Pat. No. 2,953,502 to Binning. One characteristic of this pervaporation is to make possible to separate, concentrate and purify an azeotropic mixture, a mixture of compounds having close boiling points, a heat decomposable mixture or the like which is difficult to treat by a conventional distillation method. Another characteristic of pervaporation is that it is not limited to a water soluble organic liquid mixture as in reverse osmosis, but it is generally applicable to a wide variety of organic liquid mixtures. Recently, various studies of this method have been specifically made and there are many reports relating to polymer membranes to be used in the method.
For example, regarding separation of an aqueous ethanol solution, U.S. Pat. No. 2,953,502 discloses a cellulose acetate homogeneous membrane and U.S. Pat. No. 3,035,060 discloses a polyvinyl alcohol membrane. However, both membranes have low separation factors. Although Japanese Patent Kokai No. 59-109204 discloses a composite membrane having a cellulose acetate membrane or a polyvinyl alcohol membrane as a skin layer and Japanese Patent Kokai No. 59-55305 discloses a polyethylene imine crosslinked composite membrane, their permeation rates or separation factors are low. In Japanese Patent Kokai No. 60-129104, there is described a membrane comprising an anionic polysaccharide. However, the material used for the membrane described in the Examples of this literature is a water soluble polymer and therefore durability of the membrane against an aqueous solution containing a low concentration of an organic compound is inferior. Then, in this literature, there is also described that the membrane is subjected to a crosslinking treatment with a sufficient amount of a crosslinking agent to render the membrane essentially insoluble in water, although it is not disclosed in the Examples thereof. However, usually, when a crosslinking treatment is effected, a permeation rate is decreased, while a separation factor is increased as shown by Comparative Examples hereinafter. In German Patent No. 3220570, although there is disclosed that a composite membrane obtained by coating a polymer of polyvinyl alcohol crosslinked with maleic acid on a polyacrylonitrile porous membrane shows very high separability, the permeation rate thereof is very low.
When separation of an organic liquid mixture is carried out by using these membranes, there are problems in practice as follows.
That is, since separation efficiency is low, desired concentration or separation can not be attained by permeation once through a polymer membrane. Therefore, a multi-stage operation is required, or it is necessary to combine pervaporation with another separation method, which causes trouble in practice. Further, an amount of an organic compound permeating through a polymer membrane (expressed by the amount of the permeate per unit membrane area, unit membrane thickness and unit time) is very small and, therefore, it is necessary to make the membrane area much larger, or to extremely thin the membrane thickness. In the former case, a larger apparatus is required for industrial practice, which increases cost of facilities. In the latter case, strength and durability of a membrane are lowered, which causes trouble in practice.
In order to solve these problems, various attempt have been made, but not yet been successful.